Plasma-based processes, such as plasma enhanced physical vapor deposition, plasma enhanced chemical vapor deposition, plasma etching, plasma immersion ion implantation, and conventional ion implantation, are used in the manufacturing of workpieces having microfeatures. During plasma processes, the plasma density and other plasma parameters must be tightly controlled to produce workpieces within specification. For example, the implant dose of an ion implanter depends on the ion density of the ion source, and the film deposition rate of a physical vapor deposition tool also depends on the ion density.
Conventional devices for measuring plasma parameters include a Langmuir probe. For example, FIG. 1 schematically illustrates a conventional plasma processing system 1 with a Langmuir probe 20. The system 1 further includes a processing vessel 2, a microwave transmitting window 4, and a microwave generator 6. The microwave generator 6 has a wave guide 8 and an antenna 10 positioned so that microwaves radiated by the antenna 10 propagate through the window 4 and into the processing vessel 2 to produce a plasma. The Langmuir probe 20 is inserted into the vessel 2 between process steps to measure plasma parameters. Specifically, a voltage is applied to the probe 20 and scanned from negative to positive while the current is measured. The plasma parameters can be extracted from the relationship between the voltage and current. For example, the ion density can be determined from the ion saturation current (also called a Bohm current IB) when the scanning voltage is negative. Specifically, the ion density ni can be calculated by the following equation when the scanning voltage is negative:
      n    i    =            2      q        ⁢                  I        B                    A        eff              ⁢                            M          eff                          kT          e                    in which IB is the ion saturation current collected by the probe 20 under a negative voltage, q is the ion or electron charge, Aeff is the effective area of the probe 20, kTe is the electron temperature in units of eV, and Meff is the effective ion mass.
The electron density, which should be generally equal to the ion density in a quiescent plasma, can be calculated from the electron saturation current when the scanning voltage is positive. Specifically, the electron density ne can be calculated by the following equation when the scanning voltage is positive:
      n    e    =            2      q        ⁢                  I        esat                    A        eff              ⁢                            M          e                          kT          e                    in which Iesat is the electron saturation current collected by the probe 20 when the positive scanning voltage equals the plasma potential VP, q is the ion or electron charge, Aeff is the effective area of the probe 20, kTe is the electron temperature in units of eV, and Me is the electron mass. The electron temperature Te and the plasma potential VP can be determined from the slope of the electron current and the knee of the electron saturation current, respectively.
One drawback of the Langmuir probe is that the probe cannot measure the plasma parameters in situ and in real time during processing because the probe interferes with the plasma. Specifically, the probe introduces contamination into the vessel and obstructs ingress and egress of the workpiece from the vessel. Another drawback of the Langmuir probe is that the probe cannot measure nonequilibrium plasma such as pulsed glow discharge or steady state plasma with a high voltage pulse. During pulsed plasma processes, the dynamic sheath of the plasma expands and may touch the probe if the probe is too close to the cathode. Therefore, the plasma parameters cannot be measured properly. Another issue is that during the high voltage pulse, the secondary electrons emitted from the cathode can be collected by the probe, which alters the current-voltage characteristics.
Yet another drawback of the Langmuir probe is the measurements can be inaccurate for several reasons. First, the probe draws current from the plasma, which causes significant perturbation in the plasma. Second, if the system includes a radio-frequency generator or magnetron assembly, the radio-frequency or magnetic interference can affect the measurements. Third, the measurements can be affected by sputtering, etching, and/or deposition phenomena depending on the plasma species and process conditions. Fourth, the probe does not measure the parameters of the plasma during workpiece processing, but rather before and/or after processing the workpiece. Accordingly, there is a need to improve the process of measuring plasma parameters.